msm: mdss: abstract the dsi command utility API

The dsi command parsing and construction functionalities
are hardware independent.  This is to abstract these
utility API, so that it can be shared with different versions
of  dsi host controllers.

Change-Id: I8481d9fd21e5eacc4542c72c064fa37d7fd3a11b
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>